This invention relates to a pneumatic relay and in particular a relay adapted to control the position of a damper in a unit ventilator and more specifically the invention comprises an improvement of the invention disclosed in U.S. Pat No. 3,605,781.
In a unit ventilator control circuit, the outside air damper is typically closed until the rising space temperature enters the throttling range of the room thermostat. At this point the valve controlling the flow of heating media, which has heretofore been open, begins to close, and the damper is opened to a position to take in a fixed minimum percentage of outdoor air. This minimum percentage of outdoor air is maintained throughout the lower part of the throttling range as the valve is being closed. When the temperature begins rising through the upper part of the throttling range, the valve is closed completely and the percentage of outdoor air is gradually increased to 100% which corresponds to a fully opened damper. Many schemes have been divised to provide this hesitation action of the damper position; the Mueller U.S. Pat. No. 3,115,147 discloses one such scheme. The subject invention, however, provides a simpler and more compact and economical method of providing the minimum position hesitation functions for the damper position than know prior art devices.